Intensified radioactive sources and method of preparation



April 30, 1968 H. M. DE ANGELIS 3,380,853

INTENSIFIED RADIOACTIVE SOURCES AND METHOD OF PREPARATION Original Filed Sept. 12, 1963 INVENTOR. HENRY M. DE GELIS BY W ATTOR N EYS United States Patent 3,380,853 INTENSIFIED RADIOACTIVE SOURCES AND METHOD OF PREPARATION Henry M. De Angelis, Westwood, Mass., assignor to the United States of America as represented by the Secretary of the Air Force Original application Sept. 12, 1963, Ser. No. 308,607, now Patent No. 3,326,178. Divided and this application July 1, 1964, Ser. No. 379,770

4 Claims. (Cl. 117-212) ABSTRACT OF THE DISCLOSURE Substrate material is subjected to positive ion bombardment from a radioactive vapor. A protective layer of substrate or other material, capable itself of entrapping radioactivity, is deposited and thereafter bombarded by positive ion bombardment. The process is repeated until a required intensity is reached.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The present invention relates to laminated radioactive sources and a method for their preparation, and is a division of application Ser. No. 308,607, now Patent No. 3,326,178 filed in the Patent Office Sept. 12, 1963.

These sources are prepared for four general classes of use:

(1) For reference sources;

(2) For test sources in radiation detection equipment;

(3) For spark-gap, glow lamp, and cold cathode tubes;

and

(4) For light sources, such as watch dials, etc.

It is to be understood however that the invention is not limited to these specific uses of the radioactive source specimens produced.

The object of the invention is the provision of stratified or laminated radioactive sources and a device wherein, and a method whereby, such radioactive sources can be prepared with ease, and wherein there is accurate control of intensity of radioactivity in the resulting laminated radioactive source product, and wherein the pattern or geometry of radioactivity produced in the target or source can be easily controlled.

A further object of the invention is the production of radioactive sources of increased and controllable intensity and the means and method of producing these sources, wherein substrate material is subject repetitively to alternate radioactive bombardment and deposition of substrate material, itself capable of receiving and absorbing positive ions when subjected to positive ion bombardment.

A still further object of the invention is the provision of a single unit of vacuum equipment wherein the alternating processes of radioactive bombardment and deposition of material over the bombarded substrate can both take-place without transfer or removal of the substrate from one system to another.

In this process substrate materials are bombarded with positive ions of radioactive species. During positive ion bombardment some of the ions penetrate the surface layers and are trapped. After a certain bombardment time, the surface layers are saturated with the trapped bombarding atoms. The saturation bombardment time depends upon the positive ion current density, while the saturation concentration of trapped atoms depends on the mass and energy of the bombarding ions and on the characteristics of the substrate material. Therefore, the concentration of trapped atoms and thereby the radioactive in- "ice tensity is limited. The present invention has succeeded in producing a final product source of an intensity increased beyond these limits. The entrapped radioactive atoms are prevented from escaping by depositing a thin protective layer of the substrate material (or a different substrate material) over the bombarded layer.

The protective layer thus produced is then subjected to positive ion bombardment. The time of bombardment is controlled to produce radioactive intensity of each bombarded layer to saturation or less, as desired. The bombardment can be accomplished in an apparatus in which either gaseous or vaporized solid species may be ionized and accelerated to bombard the substrate material. Where electroplating is used, it is necessary to remove the target from the vacuum system for the electroplating process, and return it for further bombardment. For vacuum deposition, however, the invention provides a device where both bombardment and substrate deposit over the bombarded layer can be carried out in a single system without the physical removal of the target from one system to another.

The term substrate material, as used herein, includes any material capable of receiving and absorbing positive ions by positive ion bombardment. Examples of metals successfully used are nickel, germanium, platinum, aluminum, tungsten, and molybdenum. Some of these can be deposited by vacuum deposit. Others may be deposited by electroplating.

The intensity of each layer is additive in the final product. It will thus be seen that sources of heightened intensities become possible. Target materials other than metals may be used such as Lucite, glass or other suitable insulating materials. The insulating material would be entirely coated with a conductor and then treated in the manner of the above described substrates. The unbombarded metallic coating may then be removed. The pattern, that is, the source geometry can be controlled by using suitable masks over the substrate material which define the area to be subjected to bombardment and protect the remaining area from exposure.

Other advantages, features and objects of this invention will be developed relative to the following description taken in connect with the accompanying drawings wherein:

The figure in the drawing is a highly enlarged view in cross section of a source specimen produced by the present invention, illustrating also the process by which it was produced.

Referring more in detail to the drawing, the target 10, shown with a high degree of magnification in the drawing, is of suitable substrate target material such as nickel, germanium, silicon, platinum, or other metals having the necessary characteristics. It is introduced into a system or an environment where radioactive material can be deposited on the base 10. The environment found to be best suited is a deposit of radioactive material in vacuum. The vacuum conditions used are of the order of from less than 1 to 5 l0- mm. Hg. Radioactive material, such as, kryptonis then introduced to pressures of the order of 1 to 2 microns. A discharge voltage of the order of 40 v. and discharge currents up to ma. provide substrate positive ion current densities of the order of from 500 to 600 microamperes per square centimeter when a potential of 1000 volts is applied between the target sub strate 10 and an anode not shown. Saturation of the surface layers of the substrate with krypton under these conditions of bombardment is reached in approximately 10- 15 seconds. It is to be understood that the specific pressure, time, voltage, and current quantities given are by Way of example only; the invention not being limited to these specific parameters.

When a predetermined degree of saturation has taken place, the target is then either removed and placed in a suitable environment for metal deposition over the saturated substrate, or the environment surrounding the target is itself changed. Vaporized aluminum, molybdenum, tungsten or other metal having the required characteristics are vaporized and deposited by vacuum deposit on the target. Suitable standard means of heating and vaporizing the metals may be employed. Applicant has developed a device which is the subject matter of the above noted parent case, where the environment is changed without removing or otherwise disturbing the target. The target is then returned to the bombarding environment where activation of a surface layer of the newly acquired substrate layer is obtained. In repeated bombardments and deposits of this nature, a radioactive source has been obtained equivalent to approximately two microcuries per cm.

The figure shows the process in schematic form. A layer B of the original substrate A reaches saturation, or any lesser degree of radioactivity desired. A layer of substrate represented at C is then deposited over the radioactive layer B. The surface of the layer C is then bombarded, producing a radioactive surface layer D, which is in turn covered with a substrate deposit E, and so on, until the desired intensity is reached.

The radioactive source specimens formed by the method of the invention have been found to have a high degree of thermal stability. Aluminum bombarded with krypton at 1000 v. was heated in vacuum to 600 C. After ten minutes at 600 C. the aluminum still retained approximately 50% of the originally trapped krypton. With ion bombardment on aluminum, krypton-85 sources of an intensity of 30 microcuries per sq. cm. can be prepared with five bombardments and five aluminum platings.

The intensity of the sources obtained can be controlled in three ways: (1) the conditions of bombardment may be controlled; i.e., discharge voltage and current, bombardment voltage and current, pressure of radioactive species and bombardment time (2) the specific activity of the bombarding species, and (3) the number of layers of bombardment and deposit.

Further, the geometry or configuration of the sources obtained can be controlled by positioning appropriate masks over the bombarded material, both the first substrate layer and the subsequent stratified or laminated layers to define the area to be bombarded. Electrostatic and/or magnetic control of the positive ions can also be used to define the area to be activated.

Thus, there has been presented an apparatus for pro ducing a radioactive source wherein each successive layer is additive with respect to the total amount of radioactivity to be produced by the source and reduced by the shielding effect of the substrates. When gases are utilized, the successive layers provide for maintenance of the entrapment, and with certain leaky materials such as nickel, an unbombarded layer may be provided as a cap.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

What I claim is:

1. The method of preparing intensified radioactive sources of known radioactive intensity, said method comprising subjecting substrate material to positive ion bombardment from a radioactive vapor to provide on said substrate a radioactive layer and thereafter entrapping the radioactivity of the layer thus produced by depositing thereover a covering layer of material capable itself of absorbing radioactivity by positive ion bombardment, subjecting the layer so deposited to positive ion bombardment, and repeating covering and bombardment steps alternately until the desired radioactive intensity is achieved.

2. The method of preparing laminated radioactive sources, said method comprising subjecting substrate material to positive ion bombardment by ionizing and accelerating kryptonunder vacuum conditions, thus producing on said substrate material a radioactive surface layer, preventing the escape of the radioactive ions from said surface layer by depositing thereover a layer of substrate material, subjecting said substrate layer to positive ion bombardment, thus producing on said layer a surface layer of entrapped radioactive ions, preventing the escape of said last-named entrapped radioactive ions by deposition of substrate material as before, repeating these steps until the desired radioactive intensity is reached.

3. The method of preparing laminated radioactive sources, said method comprising masking portions of substrate material with radioactively inert substances to provide a designed pattern of exposed substrate, subjecting said substrate material to positive ion bombardment by ionizing and accelerating krypton-85 under vacuum conditions thus producing on said substrate material a radioactive surface layer in a designed pattern preventing the escape of the radioactive ions from said surface layer by depositing thereover a layer of metallic substrate material, masking said substrate material as before, subjecting said last-named substrate layer to positive ion bombardment, thus producing on said layer a surface of entrapped radioactive ions, preventing the escape of said lastnamed entrapped radioactive ions by deposition of substrate material as before, repeating these steps of masking, bombarding, and metal deposition until the desired radioactive intensity is reached.

4. An intensified radioactive source comprising a body of plastic material coated completely with a metal, at least one side of said metal coated body having a plurality of alternate layers comprising a layer rendered radioactive by positive ion from a radioactive vapor and a layer deposited thereon of radioactivity-entrapping substrate material, capable itself of receiving and absorbing positive ions by positive ion bombardment.

References Cited UNITED STATES PATENTS 2,479,882 8/1949 Wallhauser et al. 252301 X 2,789,240 4/1957 Cohen 252-301 X 2,891,203 6/1959 Thornton 117-200 2,996,443 8/1961 Schaer 252-301 X 3,125,418 3/1964 Wendorf 117-220 X RALPH S. KENDALL, Primary Examiner.

ALFRED L. LEAVITT, Examiner.

A. GOLIAN, Assistant Examiner. 

